Solar cells may be formed from silicon wafers fabricated by a “ribbon pulling” technique. The ribbon pulling technique generally uses a crystal growth system that includes a specialized furnace surrounding a crucible containing molten silicon. The furnace generally includes a base insulation that surrounds the crucible, and insulation (known as an “afterheater”) positioned above the base insulation and near the growing ribbon crystal.
During the growth process, two strings are typically passed through the crucible so that molten silicon solidifies to its surface, thus forming a growing ribbon crystal between the two strings. Two or more ribbon crystals may be formed at the same time by passing multiple sets of strings through the crucible. The strings with the ribbon crystal attached are passed through the afterheater so that the ribbon crystal may cool in a controlled environment. The ribbon crystal is then removed from the furnace.
Care is usually taken throughout the process to reduce the amount of unwanted impurities or contaminants that are present in the ribbon crystal. Contaminants may detrimentally affect the properties of the ribbon crystal, which may impact the performance of devices made with such ribbon crystals. For example, contaminants can undesirably reduce the conversion efficiency of a solar cell made from such a ribbon crystal.